Wastewater treating processes usually include multiple treatment areas or zones which can be roughly broken down into: (1) a preliminary treatment area; (2) a primary treatment area; and (3) a secondary treatment area.
The wastewater treatment process begins with the preliminary treatment area. Preliminary treatment is concerned with removing grit and damaging debris, such as cans, bath towels, etc., from the untreated wastewater. This is usually a two-stage treatment process whereby the debris such as rags and cans are removed by screens and the grit and heavier inorganic solids settle out of the untreated wastewater as it passes through a velocity controlled zone. The damaging inorganic debris is thus removed by screening or settling while organic matter carried within the fluid stream passes on.
Following the preliminary treatment area, the wastewater is directed to a primary treatment area. The primary treatment area entails a physical process wherein a portion of the organics are removed by flotation or sedimentation. The organics removed include feces, food particles, grease, paper, etc. and are technically defined as suspended solids. Usually 40-70% of the suspended solids are removed in this primary stage.
The third treatment stage is called secondary treatment and is usually a biological treatment process where bacteria are utilized under controlled conditions to remove nutrients or nonsettling suspended and soluble organics from the wastewater. These materials would result in an unacceptable biological oxygen demand (BOD) if left untreated. Typically, one mode of this process consists of a basin in which the wastewater is mixed with a suspension of microorganisms. This mixture is then aerated to provide oxygen for the support of the microorganisms which may then adsorb, assimilate, and metabolize the excess biological oxygen demand in the wastewater. After sufficient retention time, the mixture is then introduced into a clarifier or settler into which the biomass separates as settled sludge from the liquid. The purified fluid then overflows into a receiving stream.
There are three principal types of secondary treatment for effecting treatment of wastewater. The first type, known as a trickling filter, allows the wastewater to trickle down through a bed of stone whereby the organic material present in the wastewater is oxidized by the action of microorganisms attached to the stone. A similar concept is the RBC or rotating biological contactor wherein the biology is attached to the media which rotates in the wastewater and purifies it in the manner of a trickling filter. The second method is an activated sludge process in which the wastewater is fully aerated and agitated by either compressed air or mechanical means together with a portion of the biomass which has been returned from the clarifier or settler. The third process may be referred to as a semi-aerobic (anaerobic/oxic) process in which the first stage is anaerobic or anoxic and is followed by an oxic stage. This anaerobic-oxic-anoxic process is very similar to the initial stages of the Phoredox process and the modified Bardenpho process, both well known in the wastewater treatment industry.
This anaerobic-oxic process was first disclosed in U.S. Pat. Nos. 2,788,127 and 2,875,151 to Davidson which issued in 1957 and 1959, respectively. In the anaerobic-oxic process, the untreated wastewater is first subjected to anaerobic treatment and then to aerobic decomposition. A portion of the sludge formed during the aerobic decomposition is recycled back and mixed with the untreated wastewater being subjected to anaerobic treatment. Davidson noted that the aerobic organisms in the recycled activated sludge are not impaired by passage through the anaerobic reactor and may, in fact, undergo unusual stimulation. Heidi and Pasveer confirmed the work of Davidson in 1974 and found that soluble BOD.sub.5 removal occurred in the anaerobic zone.
In recent years, there has been a great deal of work directed at biological processes for removing pollutants such as phosphorus and nitrogen (TKN) from wastewater. This work has in large part been broadly based and has not focused on specific problems and concerns. For example, many wastewater facilities are now facing very stringent phosphorus control standards. When there is already a wastewater treatment facility in place, it becomes prudent to consider the possibility of modifying these existing facilities in order to meet new standards being imposed. Obviously costs, both initial and operating, are of main concern. One important concern then is to evaluate the economics of modifying existing treatment facilities to accomplish biological phosphorus removal.
Beyond the problem of modifying an existing wastewater facility to accomplish effective and efficient biological phosphorus removal, there are certain unique or special problems that can be introduced into the process simply because of the geographical location of the wastewater treating facility and, the particular biological process currently being practiced. These special problems have not been addressed. In this regard, there are certain situations where the wastewater that is being subjected to secondary treatment has a relatively low BOD to phosphorus ratio, that is, a ratio within the range of 7-14. This presents a special problem in biologically removing phosphorus from such wastewater. It is generally appreciated that the higher the BOD to phosphorus and BOD/TKN ratio the easier it is to biologically remove phosphorus from wastewater. Thus, in some geographical locations, where the BOD content of the wastewater is relatively low, it is more difficult to create a favorable environment for the phosphorus consuming microorganisms and consequently, it is more difficult to biologically remove phosphorus. The difficulty is so pronounced that some commercially available processes that claim to biologically remove phosphorus from wastewater will not even warrant their process in wastewater conditions where there is such a relatively low BOD to phosphorus and TKN ratio. The Bardenpho process requires a BOD.sub.5 /TKN ratio of 6:25 or higher and the UCT process requires more than 3.6:1.0 ratio, preferably 5 or higher to assure phosphorus removal.
Therefore, there is a need for a biological phosphorus removal process that is particularly designed and suitable for incorporation into an existing conventional wastewater facility. Further, there is a need for an efficient and effective biological phosphorus removal process that is capable of working with wastewater that has a relatively low BOD to phosphorus and TKN ratio due to pretreatment by a fixed film reactor, chemical pretreatment, or influenced by low BOD.sub.5 /TKN and low BOD.sub.5 /TP industrial wastes.
There are certain geographical areas that also require that discharges meet certain nitrogen limits, typically 3-5 mg/l. Furthermore, it is expected that many wastewater facilities will be required to comply with nitrogen effluent limits in the future. This is because there is a growing concern about the nitrate level of wastewater dumped into receiving streams. In particular, there is growing government and public concern over the quality of drinking water as the nitrate level in the water has the potential to kill wildlife as well as to be a health risk to humans consuming such water.
To control the level of nitrogen in the wastewater effluent, one has to direct his attention to controlling the level of nitrate in the wastewater effluent. This is because of the nature of wastewater influent and conventinal processes utilized to treat wastewater. Typically, most wastewater facilities and their treatment processes will be designed to remove and control ammonia nitrogen, NH.sub.4. In the process of doing that, nitrates will be formed and will exist in the discharge. Consequently, to control nitrogen, a process must be directed at removing nitrites and nitrates, NO.sub.2 and NO.sub.3.
Fundamentally to do this, the process must include a vehicle for removing oxygen from the nitrate compound so that the associated nitrogen can escape to the atmosphere.
There are physical and chemical processes for removing ammonia nitrogen and nitrates from the wastewater. But these processes have serious drawbacks in that they tend to be prohibitively expensive and beyond that, they often are found to be most incompatable with co-existing processes for removing other pollutants from the same wastewater. It is not unusual for such a nitrogen removing process to have such an adverse affect on another co-existing pollutant removal process that that process becomes totally ineffective and consequently the entire process fails.
For some time now, many wastewater facilities having been moving to biological processes for removing the more conventional pollutants of concern such as phosphorous, ammonia nitrogen, BOD, suspended solids, etc. It is widely accepted by many authorities in the wastewater treatment fields that biological processes are the most economical and practical way of treating wastewater in most situations. That is supported by the great number of wastewater facilities that have been designed to carry out biological wastewater treatment in recent years.
There have been attempts at biologically controlling nitrogen by incorporating an anoxic zone downstream from a series of preceeding zones that would typically include aerobic an anerobic zones. In order to remove and control those pollutants traditionally considered of prime importance, such as ammonia nitrogen, BOD, and phosphorous, these biological processes require that the anaerobic and oxic zones be disposed in the initial stages of treatment. Consequently, by the time the wastewater or mixed liquor has reached the downstream anoxic zone, there is very little, if any, food source for the microorganisms and without food the effectiveness of downstream denitrification is seriously hampered and usually inefficient. Besides that, the overall effectiveness of such a biological denitrification process depends on flow and the overall makeup of the wastewater which can vary sharply from time to time.
Therefore, there is a need for a biological denitrification process that itself can be controlled so as to vary and maintain the food level within the anoxic zone at an appropriate level to effectively denitrify to a level of approximately 3 milligrams per liter irrespective of flow, wastewater makeup, or any other factor that might affect denitrification.